Main Notes:
Water Potential (Ψw) Water potential is a measure of the free energy per unit volume of water. It determines the direction of water movement — water always flows from higher Ψw to lower Ψw (down the free energy gradient).
Equation: Ψw = Ψs + Ψp
| Component | Symbol | Value | Condition |
|---|---|---|---|
| Solute potential (osmotic) | Ψs | Always ≤ 0 | More solutes = more negative |
| Pressure potential (turgor) | Ψp | Positive in turgid cell | Zero at incipient plasmolysis |
| Pure water reference | Ψw | 0 | Standard reference point |
Key States of a Plant Cell:
- Fully turgid: Ψp is at maximum, Ψw = 0 (if in pure water)
- Incipient plasmolysis: Ψp = 0, Ψw = Ψs
- Plasmolysed: Ψp = 0 or negative (protoplast detached from wall)
Osmosis: Passive movement of water from high Ψw → low Ψw across a semipermeable membrane. Requires: (1) semipermeable membrane, (2) Ψw gradient.
Imbibition: Adsorption of water by hydrophilic colloids. Does NOT require semipermeable membrane. Example: dry seed swelling.